THE huge mushroom cloud soared skywards, the captain was gripped by fear, believing his plane was about to be engulfed by the fall-out from a nuclear explosion. After declaring mayday and ordering his crew to don oxygen masks, the experienced pilot had the presence of mind to record that the cloud measured an estimated 200 miles in diameter and was tipped by an eerie light, like nothing he had seen before. Eventually, it soared harmlessly into the atmosphere, leaving the passenger jet to continue safely on its journey from Anchorage, in Alaska, to Tokyo.

But far below, a fleet of fishing boats trawling the sea between Japan and the Soviet Union was drenched by a violent but short-lived downpour before the weather suddenly cleared. Nuclear tests and volcanic activity were later ruled out but scientists concluded that this was not a natural phenomenon. More than two decades later suspicion still exists that the stunned airline crew and fishermen in 1973 were witnessing a sinister Cold War experiment, in which water from the Sea of Japan was blown into the air to create clouds and rain.

British government papers, just released by the National Archives, show that throughout the Seventies there was deep mistrust between the two superpowers over environmental warfare. The documents reveal that both the US, which led the field, and the Soviet Union had secret military programmes with the goal of controlling the world's climate. "By the year 2025 the United States will own the weather, " one scientist is said to have boasted.

Since then, a United Nations treaty has been signed which bans environmental warfare, such as causing earthquakes, melting the polar ice caps and altering climate. But some experts believe that clandestine work to create the ultimate weapon of mass destruction continues.

These claims are dismissed by sceptics as wild conspiracy theories and the stuff of James Bond movies but there is growing evidence that the boundaries between science fiction and fact are becoming increasingly blurred. The Americans now admit that they invested L12million over five years during the Vietnam war on "cloud seeding" - deliberately creating heavy rainfall to wash away enemy crops and destroy supply routes on the Ho Chi Minh trail, in an operation codenamed Project Popeye.

It is claimed that rainfall was increased by a third in targeted areas, making the weather-manipulation weapon a success. At the time, government officials said the region was prone to heavy rain.

However this sort of rain-making experiment was nothing new. In Britain, it has been alleged that before the devastating Lynmouth floods in Devon in 1952, the RAF had been conducting secret rain-making tests. Aircraft showered clouds with silver iodide, on which water droplets formed, became heavy and eventually fell to the ground as rain. In the next 12 hours nine inches of rain fell - 250 times the normal amount for August - and 35 people were killed.

Former North Devon MP Tony Speller, then a 22-year-old soldier who helped in the relief effort, sought answers from the MoD.

"I have no doubt they were seeding in the area because there were RAF log books to prove it, " he says now. "Of course the MoD denied any knowledge but that is not to say it did not happen."

Speller, now 76, adds: "I doubt we will ever know the truth."

Early work on climate control was crude and unpredictable but it is claimed that both the Americans and Russians continued to experiment behind closed doors even after the UN ban in the mid-Eighties, and both now possess sophisticated systems which are capable of controlling the weather - with potentially devastating results.

In the US, the technology was developed under the high-frequency active auroral research programme ( HAARP) - originally part of Ronald Reagan's controversial Star Wars defence system. Based in Gokoma, Alaska, the weapon operates by beaming powerful radio waves into the upper atmosphere to alter weather patterns. Some experts claim the system is already up and running, while others say it won't be ready for another 20 years.

Michel Chossudovsky, professor of economics at the University of Ottawa in Canada, who has studied official military documents about HAARP, is in no doubt that the weapon is ready.

"There are very clear statements by the US Air Force to the effect that weather modification technology is available. HAARP will be fully operational by next year and could be used in actual military situations, " he says.

"To claim this system has any nonmilitary purposes is twisting the truth.

I don't think there are any peaceful applications - it is a weapon of mass destruction, capable of major climatic disturbance. Part of the beauty is that the enemy might never know that a weapon had been used. I believe the UN agreement is certainly being violated."

He claims that at least one British firm has been involved in its development.

"It is time people began focusing on these weapons instead of concentrating solely on global warming, " Chossudovsky adds. "Both are a serious threat."

The Russians are thought to have their own "weather steering" system, called Woodpecker, involving the transmission of low-frequency waves which are capable of disrupting the atmosphere and altering the path of the jet stream. It is claimed that a prolonged drought in California in the Eighties was caused by the blocking of warm, moist air for many weeks.

According to Damian Wilson, a physicist with the Met Office, controlling climate is a reality but not a precise science. "Clearing fog by dropping dry ice into clouds is a proven technique which has been around for decades, " he says.

"Large amounts of research have been invested in seeding clouds to generate rain and it is done in countries where there are water shortages. The problem is that it is unpredictable and you need clouds to start with. The technology does not exist to make rain fall from clear blue skies so it cannot be used in the desert to end droughts and famines."

WILSON believes it is possible to alter the course of a hurricane, which could have enormous life-saving potential. The current mayhem in the Caribbean and America's Eastern seaboard also shows what a destructive weapon a well-targeted storm could prove.

The Americans used cloud seeding to try to control a hurricane in 1947 but the tactic backfired when it picked up strength and hit Savannah, Georgia.

It is known that the US carried out further hurricane-manipulation experiments between 1962 and 1983, under the codename Project Stormfury, after it was calculated that a single hurricane contained as much energy as all the world's power stations combined. More recent projects have involved pouring tens of thousands of gallons of vegetable oil on to the sea.

"Hurricanes gather their strength from the warm sea surface, " says Wilson. "By spreading a large film of oil on the sea it would reduce the intensity by cooling the surface. In theory it is possible to change the path of the hurricane this way. It would not surprise me if military research into controlling the weather goes on. As we suffer more summer droughts in the south-east of England I would also expect to see pressure for cloud seeding to be introduced in this country."

It is not just the weather that has attracted the attention of the military.

Scientists have also researched ways of triggering earthquakes. By setting off small quakes, pressure could be released and a disaster averted. But military scientists believe it is also possible to direct powerful energy beams into vulnerable fault zones, causing the Earth's plates to shift, creating a massive earthquake. Along fault lines beneath the oceans, the same technology could be used to launch devastating tsunamis.

Part of the problem in banning experiments involves agreeing a definition of what environmental warfare is. It has been argued that the famous Dambusters mission during the Second World War, when bouncing bombs were used to flood the German industrial heartland, were a form of environmental warfare.

Half a century later, the threat is still being taken sufficiently seriously.

Former US defence secretary William Cohen warns: "Terrorists are engaging even in an eco-type of terrorism whereby they can alter the climate, set off earthquakes and volcanoes remotely through the use of electromagnetic waves. It's real, and that's the reason why we have to intensify our efforts."

Owen Greene, director of research at Bradford University's department of Peace Studies, believes efforts are continuing in secret to develop weapons that harness nature.

"There is so much money within the Pentagon that it would surprise me if they were not looking into it, " he says. "I suspect it is going on in both the US and Russia. You can't stop people researching the weather. As for earthquakes, you don't even need to spend money on active military programmes because there is so much other work going on which could easily be adapted. Some of the ideas are quite credible."

Another defence source says: "The risk is that by fooling around with nature, we may unleash irreversible damage and change our entire planet's atmosphere."

Despite these stark warnings, such is the immense power of nature, it seems inevitable that Man will continue to play God.